A. Technical Field
The present invention relates to a water-absorbing agent and its production process and use, more particularly, a water-absorbing agent and its production process and an absorbent article wherein the water-absorbing agent is favorably used not only for sanitary materials such as disposable diapers, sanitary napkins, and incontinent pads, but also in various industrial fields such as public works, agriculture, and horticulture.
B. Background Art
In recent years, water-absorbing agents comprising water-absorbent resins are widely utilized in fields, for example, of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Known examples of the above water-absorbent resins are as follows: crosslinked polymers of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylonitrile graft polymers; neutralized products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked polymers; and crosslinked polymers of cationic monomers.
Examples of the properties which the above water-absorbing agents should have are excellent properties such as absorption capacity or speed and liquid-permeability upon contact with aqueous liquids such as body fluids. However, relations between these properties do not necessarily display positive correlations. For example, as the absorption capacity increases, properties such as liquid-permeability or absorption speed deteriorate.
As to a method for improving such water-absorption properties of the water-absorbing agent in good balance, there is a proposed art in which the neighborhood of the surface of the water-absorbent resin is crosslinked with crosslinking agents such as polyhydric alcohols. In addition, there are also known methods in which when the crosslinking agent is added the following materials are allowed to be present as third substances, thus attempting to more uniformly distribute the crosslinking agent on the surface of the water-absorbent resin to uniformly carry out the surface-crosslinking in the course of the above crosslinking reaction: inert inorganic powders; dihydric alcohols; ether compounds; water-soluble polymers; and alkylene oxide adducts of monohydric alcohols, or organic acid salts, or lactams (for example, JP-A-56-159232, JP-A-60-036534, JP-A62-036411, JP-A-05-031362, JP-A-05-507511, JP-A-06-184320, JP-A-08-084927).
However, because most of crosslinking agents or treatment solutions containing them, as actually used, are liquid compounds, they have high affinity for the water-absorbent resin, and their reaction or absorption to the resin starts simultaneously with their mixing with the resin, so it is difficult to uniformly disperse them onto the surface of the water-absorbent resin or to uniformly produce a surface-crosslinked layer, and it is also impossible to freely control the extent of the crosslinking in the surface neighborhood. In addition, the above conventional surface-crosslinking may be able to prevent the liquid permeability from depressing, but still deteriorates the absorption capacity. In addition, there are reported methods in which inorganic powders or powdered denaturing agents (as obtained by powdering of denaturing agents such as liquid crosslinking agents) are attached to the surface of water-absorbent resin particles (JP-A-61-257235, JP-A-09-278900), or crosslinkages between water-absorbent resin particles are formed with crosslinking agents, thereby granulating and aggregating the resin particles. These methods may also improve the liquid permeability, but are still insufficient in respect to the improvement of the absorption properties.